This invention relates to demand or impulse ink jets of the type wherein droplets of ink are fired at varying time intervals as required or demanded for printing.
A demand or impulse ink jet comprises an ink jet chamber including an orifice from which droplets are ejected and an ink supply inlet coupled to a source or reservoir of ink. In such a jet, a driver is coupled to the chamber such that the change in state of energization of the driver results in the ejection of a droplet of ink from the orifice. By changing the state of energization of the driver at the appropriate moment, a droplet of ink may be ejected on demand in a trajectory directed at a suitable substrate or target such as paper.
Copending applications Ser. No. 336,603, filed Jan. 4, 1982 and Ser. No. 576,582, filed Feb. 3, 1984 disclose an ink jet apparatus comprising an array of demand or impulse ink jets where each jet ejects a droplet of ink from an orifice in response to the expansion and contraction of drivers in the form of elongated transducers which are energized in response to a field selectively applied transversely to the axis of elongation. As a result, substantial displacement of the transducers is achieved which is capable of ejecting a droplet of ink.
Heretofore, demand or impulse ink jets of this type have employed an ink which is stored in a reservoir in a liquid state supplied to one or more ink jet chambers in a liquid state and ejected from the chambers through the orifice in a liquid state. Upon ejection, the droplets dry on the target which receive the droplets over a certain length of time which is a function of the particular ink utilized.
Inks utilized in such demand or impulse ink jets may be of various types. Water-based inks may be used with demand ink jets and may be incompatible with paper having poor penetration and drying characteristics with respect to the paper. On the other hand, oil-based inks which are more compatible with paper, have low surface tension and generally provide ink dots which are too large to give good print quality. Inks stored in a liquid state whether oil or water-based may undergo some deterioration, evaporation and degeneration including the possible pick-up of dirt, dust and other particulate matter. Moreover, liquid inks which are pigmented may undergo settling and agglomeration. The shelf-life may, therefore, be limited.
In continuous ink jets wherein droplets are continuously ejected at regular intervals and deflected where not desired, inks have been employed which undergo a phase change. When ink is stored within a reservoir supplying an ink jet chamber continuously ejecting well-formed spherical droplets, the ink must be heated. Liquid ink is then ejected and cooled after striking a suitable target. In such a continuous ink jet, the ink is constantly under pressure and droplets are continuously ejected from the orifice over some distance and length of time so as to permit cooling to the phase change temperature before striking the paper. See, for example, U.S. Pat. Nos. 3,715,219 and 3,653,932 for a disclosure of a continuous jet with a phase change ink.